Molecular adsorption and self-diffusion of NO2, SO2, and their binary mixture in MIL-47(V) material

The loading dependence of self-diffusion coefficients (Ds) of NO2, SO2, and their equimolar binary mixture in MIL-47(V) have been investigated by using classical molecular dynamics (MD) simulations. The Ds of NO2 are found to be two orders of magnitude greater than SO2 at low loadings and temperatures, and its Ds decreases monotonically with loading. The Ds of SO2 exhibit two diffusion patterns, indicating the specific interaction between the gas molecules and the MIL-47(V) lattice. The maximum activation energy (Ea) in the pure component and in the mixture for SO2 are 16.43 and 18.35 kJ mol-1, and for NO2 are 2.69 and 1.89 kJ mol-1, respectively. It is shown that SO2 requires more amount of energy than NO2 to increase the diffusion rate. The radial distribution functions (RDFs) of gas-gas and gas-lattice indicate that the Oh of MIL-47(V) are preferential adsorption site for both NO2 and SO2 molecules. However, the presence of the hydrogen bonding (HB) interaction between the O of SO2 and the H of MIL-47(V) and also their binding angle (θ(OHC)) of 120° with the linkers of lattice indicate a stronger binding interaction between the SO2 and the MIL-47(V), but it does not occur with NO2. The jump-diffusion of SO2 between adsorption sites within the lattice has been confirmed by the 2D density distribution plots. Moreover, the extraordinarily high Sdiff for NO2/SO2 of 623.4 shows that NO2 can diffuse through the MIL-47(V) significantly faster than SO2, especially at low loading and temperature.

Capture and Separation of SO2 Traces in Metal-Organic Frameworks via Pre-Synthetic Pore Environment Tailoring by Methyl Groups

Herein, we report a pre-synthetic pore environment design strategy to achieve stable methyl-functionalized metal-organic frameworks (MOFs) for preferential SO2 binding and thus enhanced low (partial) pressure SO2 adsorption and SO2 /CO2 separation. The enhanced sorption performance is for the first time attributed to an optimal pore size by increasing methyl group densities at the benzenedicarboxylate linker in [Ni2 (BDC-X)2 DABCO] (BDC-X=mono-, di-, and tetramethyl-1,4-benzenedicarboxylate/terephthalate; DABCO=1,4-diazabicyclo[2,2,2]octane). Monte Carlo simulations and first-principles density functional theory (DFT) calculations demonstrate the key role of methyl groups within the pore surface on the preferential SO2 affinity over the parent MOF. The SO2 separation potential by methyl-functionalized MOFs has been validated by gas sorption isotherms, ideal adsorbed solution theory calculations, simulated and experimental breakthrough curves, and DFT calculations.

Multi-molar CO2 capture beyond the direct Lewis acid-base interaction mechanism

Some singly charged ionic liquids (ILs) have been reported to absorb multi-molar CO2. However, the conventional acid(CO2)-base(anion) interaction picture leads to too weak CO2 binding to support the high uptake. Later, a so-called "cation-channel" mechanism assuming the cation-to-anion proton transfer successfully explains the over equimolar CO2 uptake of some phosphonium-based ILs. Here, by employing the density functional theory (DFT) calculations, we extend the proton transfer mechanism to incorporate imidazole- and ammonium-based ILs as well. For imidazole-based ILs, carbene molecules formed after the proton transfer can react strongly with CO2. More importantly, for ammonium-based ILs, the proton transfer process is feasible only with the help of CO2 molecules. Furthermore, compared to the one IL ion pair model, the model consisting of two IL ion pairs can result in stronger CO2 absorption because it can describe the intermolecular hydrogen bonds more appropriately, especially after incorporating CO2 molecules. The relative acidity and basicity of cations and anions in ILs may be crucial for understanding their functionalization as ILs.

Exploration of tetra-branched multiple-site SO2 capture materials

Efficient exploration of the configuration space of the reaction complexes consisting of multi-branched structures and SO₂ molecules.